Heat-resistant steel

ABSTRACT

A high temperature corrosion resistant stainless steel having a composition (by weight) of: C≦0.2%, but more than zero, N≦0.1% but more than zero, O≦0.1% but more than zero, Si≦0.4% but more than zero, Al&lt;0.5% but more than zero, Mn≦0.5% but more than zero, Cr 20 to 25%, Ni≦2.0% but more than zero, Zr+Hf 0.01 to 0.1%, Ti≦0.5% but more than zero, Mo+W≦2.5% but more than zero, Nb+Ta≦1.25% but more than zero, V≦0.5% but more than zero, and balance of Fe and naturally occurring impurities and not more than 0.010% of S impurity. This steel is particularly suitable for the manufacturing of interconnects in solid oxide fuel cells.

RELATED APPLICATION DATA

This application is based on and claims priority under 35 U.S.C. §119 toSwedish Application No. 0401292-8, filed May 19, 2004, the entirecontents of which are incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to a steel product, which athigh temperatures forms an oxide scale with good surface conductivityand an excellent adhesion to the underlying steel. In particular, itrelates to a ferritic chromium steel suitable for the use asinterconnects or bipolar plates in solid oxide fuel cells or other hightemperature applications such as catalytic converters in cars andtrucks.

BACKGROUND

In the discussion of the state of the art that follows, reference ismade to certain structures and/or methods. However, the followingreferences should not be construed as an admission that these structuresand/or methods constitute prior art. Applicants expressly reserve theright to demonstrate that such structures and/or methods do not qualifyas prior art against the present invention.

Ferritic chromium steels are used for applications with highrequirements on heat resistance, such as for example interconnectmaterials in Solid Oxide Fuel Cells (SOFC) or if alloyed with Al, as amaterial for catalytic converters. They are very suitable materials foruse in SOFC applications since the Thermal Expansion Coefficients (TEC)of ferritic steels are close to the TECs of the electro-active ceramicmaterials used in the SOFC stack, such as yttrium-stabilized zirconia(YSZ) which is the common material used as electrolyte in the fuel cell.This has, for instance, been studied by Linderoth et al. in“Investigation of Fe—Cr ferritic steels as SOFC interconnect material”,Mat. Res. Soc. Symp. Proc., Vol. 575, (1999), pp. 325-330.

It is desired that the oxide scale formed on the steel interconnectmaterial does not spall off or crack due to thermal cycling, i.e., theoxide scale should have good adhesion. The formed oxide scale shouldalso have good electrical conductivity and not grow too thick during thelife time of the fuel cell, since thicker oxide scales will lead to anincreased electrical resistance. The formed oxide should also bechemically resistant to the gases used as fuels in an SOFC, i.e., novolatile metal-containing species such as chromium oxyhydroxides shouldbe formed. Volatile species such as chromium oxyhydroxide willcontaminate the electro-active ceramic materials in a SOFC stack, whichwill lead to a decrease in the efficiency of the fuel cell.

One disadvantage with the use of commercial ferritic chromium steel isthat they are usually alloyed with aluminum and/or silicon, which formAl₂O₃ and/or SiO₂ at the working temperature of the SOFC. Both theseoxides are good electrical insulating oxides, which will increase theelectrical resistance of the cell and lower the fuel cell efficiency.

This has led to the development of ferritic steels with low Al and Sicontents, to ensure good conductivity of the formed oxide scales. Thesenewly developed steels are usually also alloyed with manganese. Theaddition of Mn in the steel will induce the formation of chromium oxidebased spinell structures in the formed oxide scale. However, Mn ingeneral has a poor effect on the corrosion resistance of the steel; itis therefore desired that the Mn content in the steel be monitoredcarefully at low levels. Too high a concentration of Mn in the steelwill lead to the growth of thick oxide scales due to severe hightemperature corrosion.

In addition to Mn, several of these new developed steels are alloyedwith group III elements, i.e., Sc, La and Y and/or other rare earthelements (REM). The addition of La, Y or REM is made to increase thelifetime of the material at high temperatures. Strong oxide formers suchas La, Y and REM are said to decreases the oxygen ion mobility in theformed Cr₂O₃ scale, which will lead to a decrease in the growth rate ofthe oxide scale. The amount of added REM to the steel has to becarefully monitored, since too high a concentration of REM will lead toproduction process difficulties, as well as undesired corrosionproperties of the steel.

In patent application US 2003/0059335, the steel is alloyed with a smallamount of La (0.01-0.4% by weight) and optionally also with smallamounts of Y and Ce (0.1 to 0.4% by weight).

In patent application EP 1 298 228 A2, the steel is also alloyed witheither Y (≦0.5% by weight) or REM (≦0.2% by weight) or La (0.005-0.1% byweight).

In U.S. Pat. No. 6,294,131 B1, the steel is also alloyed with REM (0.005to 0.5% by weight) and in patent application US 2002/0192468 A1 thefinal steel is alloyed with 0.01-1.5% yttrium, rare earth metals, andoxides thereof.

In addition to these above-mentioned patents, there are somecommercially available ferritic steel for interconnects in SOFC. Two ofthese are the steel sorts A and B (see further details of A and B inExample 3 below), A being alloyed with 0.04% La, and B with a La contentof max 0.2% by weight. All the above mentioned patents and commerciallyavailable steels are alloyed with small amounts of rare earth metalssuch as Y, La and Ce. The addition of reactive rare earth metals willlead to a decrease in corrosion resistance compared with the steel alloyof this invention.

SUMMARY

It is an object of the present invention to provide a steel alloy withexcellent high temperature corrosion resistance. Another object of thepresent invention is that the oxide scale on said steel alloy has a goodadherence and a low surface resistivity. A further object of the presentinvention is that the above mentioned properties are so good that saidalloy does not need alloying with any REM or group III metals, which inturn will lead to a simpler and more cost-effective steel productionprocess. Yet another object of the present invention is to provide asteel alloy for the manufacturing of interconnects and/or bi-polarplates to Solid Oxide Fuel Cells. A further object of the presentinvention is to provide a steel alloy for the manufacturing of catalyticconverters in automobile applications.

The above objects and further advantages are achieved by carefullymonitoring the contents of different alloying elements of the steelalloy. This is done by alloying the steel with 20 to 25% by weight ofchromium and monitoring the content of oxide formers such as silicon,aluminum and manganese at low levels. In addition to this, elements,such as Ni, Mo and group IV (titanium group) and group V elements(vanadium group) of the periodic table of elements, are added to thealloy. Said alloy is produced in a conventional steel productionprocess. The final product of said alloy can have the form of a strip,foil, wire, tube, bar or even as a powder, preferably as strip or afoil.

One factor is that said alloy is heat resistant at temperatures up to900° C. and that the oxide scale formed does not grow too thick.Therefore, the mass gain per unit area of said alloy is less than 1.5mg/cm², when the steel alloy has been oxidized in air or in air +1% H₂Omixture for 1000 hours at 850° C. or any environment similar to thegases used in a Solid Oxide Fuel Cell. A further aspect is that thegrown oxide scale does not spall off, i.e., has a good adhesion to theunderlying alloy.

To be able to use the steel alloy as interconnect or bipolar plate inSOFC, the thermal expansion of said alloy should not deviateconsiderably from the thermal expansions of the anode material or theelectrolyte material used in the fuel cell. In one exemplary embodiment,said alloy has a thermal expansion coefficient of 10 to 15·10⁻⁶° C.⁻¹ inthe temperature range 0 to 900° C., or even more preferably 11 to14·10⁻⁶C.⁻¹, and most preferably 11.5 to 13·10⁻⁶° C.⁻¹. As a consequencethereof, the thermal expansion mismatch (TEM) between the electro-activeceramic materials in the fuel cell and the thermal expansion of saidalloy is not greater than ±25%, or preferably less than =20%, or mostpreferred lower than ±15%. Here the thermal expansion mismatch (TEM) isdefined as (TEC_(ss)-TE_(ce))/TEC_(ss), where the TEC_(ss) is thethermal expansion of the steel alloy and TEC_(ce) is the thermalexpansion of the electro-active ceramic materials used inanode-supported fuel cells.

Yet another important object is that said alloy has a good conductivity.The bipolar plate works as a current collector in a fuel cell. To avoiddegradation of the fuel cell efficiency, the contact resistance of thesteel alloy is kept as low as possible throughout the lifetime of thefuel cell. The area specific resistance (ASR) of said alloy in a SOFCsetup should be kept low but also the increment of ASR with time is keptas low as possible. If the increment of the ASR is large, this can leadto a decrease of the fuel cell efficiency.

In one exemplary embodiment, a high temperature corrosion resistantstainless steel, consists essentially of (by weight):

-   -   C≦0.2%, but more than zero;    -   N≦0.1% but more than zero;    -   O≦0.1% but more than zero;    -   Si≦0.4% but more than zero;    -   Al<0.5% but more than zero;    -   Mn≦0.5% but more than zero;    -   Cr 20 to 25%;    -   Ni≦2.0% but more than zero;    -   Zr+Hf 0.001 to 0.1%;    -   Ti≦0.5% but more than zero;    -   Mo+W≦2.5% but more than zero;    -   Nb+Ta≦1.25% but more than zero;    -   V≦0.5% but more than zero; and    -   balance of Fe and naturally occurring impurities and not more        than 0.010% of S impurity

In another exemplary embodiment, a high temperature corrosion resistantstainless steel consists essentially of (by weight):

-   -   C≦0.1% but more than zero;    -   N≦0.1% but more than zero;    -   O≦0.1% but more than zero;    -   Si≦0.4% but more than zero;    -   Al<0.4% but more than zero;    -   Mn≦0.4% but more than zero;    -   Cr 20 to 25%;    -   Ni≦1.0% but more than zero;    -   Zr 0.001 to 0.1%;    -   Ti≦0.4% but more than zero;    -   Mo≦2.5% but more than zero;    -   Nb≦1.25% but more than zero;    -   V≦0.1% but more than zero; and    -   balance of Fe and naturally occurring impurities and not more        than 0.010% of S impurity.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the weight gain per unit area plotted vs. time of anexemplary embodiment of a disclosed steel alloy, together with the foursteel alloys (Sandvik ID numbers 433, 434, 436 and 437) produced forcomparison, oxidized in air for 336, 672 and 1008 hours, respectively.

FIG. 2 shows a SEM cross-section micrograph of the oxide scale formed onan exemplary embodiment of a disclosed steel alloy oxidized in air for336 hours at 850° C.

FIG. 3 shows a Glow Discharge Optical Emission Spectroscopy (GDOES)depth profile of the oxide scale formed on an exemplary embodiment of adisclosed steel alloy oxidized in air for 336 hours at 850° C.

FIG. 4 shows the weight gain per unit area for eight different steelgrades including an exemplary embodiment of a disclosed steel alloy andthe four steel alloys (Sandvik ID numbers 433, 434, 436 and 437)produced for comparison after oxidization in air+1% H₂O at 850° C. after500 hours.

DETAILED DESCRIPTION

Chemical Composition: The chemical composition of the steel alloyaccording to the present disclosure comprises the following elements (byweight-%):

-   -   C≦0.2% but more than zero, preferably 0.001<C≦0.2%    -   N≦0.1% but more than zero, preferably 0.001<N≦0.1%    -   O≦0.1% but more than zero, preferably 0.001<O≦0.1%    -   Si≦0.4% but more than zero, preferably 0.01<Si≦0.4%    -   Al<0.5% but more than zero, preferably 0.001<Al<0.5%    -   Mn≦0.5% but more than zero, preferably 0.01<Mn≦0.5%    -   Cr 20 to 25%    -   Ni≦2.0% but more than zero, preferably 0.01≦Ni≦2.0%    -   Zr+Hf≦0.1% but more than zero, preferably 0.001≦Zr+Hf≦0.1%    -   Ti≦0.5% but more than zero, preferably 0.01≦Ti≦0.5%    -   Mo+W≦2.5% but more than zero, preferably 0.01≦Mo+W≦2.5% or even        more preferably 0.1≦Mo+W≦2.0%    -   Nb+Ta≦1.25% but more than zero, preferably 0.01≦Nb+Ta≦1.25%    -   V≦0.5% but more than zero, preferably 0.01≦V≦0.5%    -   and balance of Fe and naturally occurring impurities, but not        more than 0.010% of S impurity. Said alloy is produced in an        ordinary steel production process.

The chemical composition of the steel alloy according to the presentdisclosure can also comprise the following elements (by weight-%):

-   -   C≦0.1% but more than zero, preferably 0.001<C≦0.1%    -   N≦0.1% but more than zero, preferably 0.001<N≦0.1%    -   O≦0.1% but more than zero, preferably 0.001<O≦0.1%    -   Si≦0.4% but more than zero, preferably 0.01<Si≦0.4%    -   Al<0.4% but more than zero, preferably 0.001<Al≦0.4%    -   Mn≦0.4% but more than zero, preferably 0.01<Mn≦0.4%    -   Cr 20 to 25%    -   Ni≦1.0% but more than zero, preferably 0.01≦Ni≦1.0%    -   Zr≦0.1% but more than zero, preferably 0.001≦Zr≦0.1%    -   Ti≦0.5% but more than zero, preferably 0.01≦Ti≦0.4%    -   Mo≦2.5% but more than zero, preferably 0.01≦Mo≦2.5% or even more        preferably 0.1≦Mo≦2.0%    -   Nb≦1.25% but more than zero, preferably 0.01≦Nb≦1.25%    -   V≦0.1% but more than zero, preferably 0.01≦V≦0.1%    -   and balance of Fe and naturally occurring impurities, but not        more than 0.010% of S impurity. Said alloy is produced in an        ordinary steel production process.

High Temperature Corrosion Resistance: The disclosed alloy is heatresistant at temperatures up to 900° C. and the oxide scale formed uponoxidization does not grow too thick. In Table 1, the theoretical massgain per area unit for a chromium oxide scale with different thicknessesis calculated. The calculations assume that a dense and pure Cr₂O₃ scaleis formed on the surface of the steel. The Cr₂O₃ has a density of 5300mg/cm³ and the mass percent of oxygen in the Cr₂O₃ is 31.6%. This willgive a mass gain per unit area of 0.16 mg/cm² for a 1 μm thick pure anddense chromium oxide scale, and 0.82 mg/cm² for 5 μm thick oxide scale.

Noted here should be that these values of mass gain of the formation ofpure Cr₂O₃ are theoretical. When a ferritic chromium steel alloy isoxidized, usually mixed oxides are formed and the weight gain depends onthe added alloying elements. However, a low weight gain is importantsince higher weight gains will lead to thicker oxide scale formations,which in turn will increase the resistance of the steel. Exemplaryembodiments of the disclosed steel alloy therefore have a weight gain ofless than 1.5 mg/cm² after 1000 hours of exposure to air and/or air +1%H₂O at 850° C.

A further feature of the disclosed alloy is that the grown oxide scaledoes not spall off, i.e., has a good adhesion to the underlying alloy.

Thermal Expansion: To be able to use steel as interconnects or bipolarplates in SOFC, the thermal expansion of the alloy should not deviategreatly from the thermal expansion of the anode material or theelectrolyte material used in the fuel cell. Therefore, the disclosedsteel alloy has a thermal expansion coefficient of 10×10⁻⁶ to 15×10⁻⁶°C.⁻¹ in the temperature range 0 to 900° C., or even more preferably11×10⁻⁶ to 14×10⁻⁶° C.⁻¹, and most preferably 11.5×10⁻⁶ to 13×10⁻⁶°C.⁻¹. Further, the thermal expansion mismatch (TEM) between theelectro-active ceramic materials in the fuel cell and the thermalexpansion of said alloy is not greater than ±25%, preferably less than±20%, and most preferably lower than ±15%.

Here the thermal expansion mismatch (TEM) is defined as(TEC_(ss)-TEC_(ce))/TEC_(ss), where the TEC_(ss) is the thermalexpansion of the alloy and TEC_(ce) is the thermal expansion of theelectro-active ceramic materials. The thermal expansion of said steelalloy can be tuned to match the thermal expansion of the electro-activeceramic materials in the fuel cell by carefully monitoring the amount ofalloying elements, such as nickel, in the steel alloy.

Conductivity: Embodiments of the disclosed steel alloy have a goodconductivity and, in a SOFC setup, have an ASR of less than 50 mΩ cm²after 1000 hours, preferably an ASR even lower that 25 mΩ cm² after 1000hours, on both anode and cathode side of the interconnect. Moreover, theincrement of the ASR is not greater than 10 mΩ cm² per 1000 hours,preferably even lower than 5 mΩ cm² per 1000 hours on both the anode andthe cathode side of the interconnect. This factor promotes a goodefficiency of the fuel cell throughout the life time of the fuel cell,which might be as long as 40,000 hours.

A preferred embodiment of the disclosed steel alloy will now bedescribed in more detail. First, the steel alloy is produced by ordinarymetallurgical steel making routines to the chemical composition asdescribed, for example, in the following Examples. Then said steel alloyis hot-rolled down to an intermediate size, and thereafter cold-rolledin several steps with a number of recrystallization steps, until a finalspecific thickness of normally less than 3 mm, and a width of maximally400 mm. The linear thermal expansion of said steel alloy was determinedby dilatometer measurement and was found to be 12.3×10⁻⁶° C.⁻¹ for thetemperature range 30 to 900° C.

EXAMPLE 1

A 0.2 mm thick steel alloy strip with a nominal composition (by weight)of max 0.2% C, max 0.1% N, max 0.1% 0, max 0.4% Si, max 0.5% Al, max0.5% Mn, 20 to 25% Cr, max 2.0% Ni, 0,001 to 0.1% Zr+Hf, max 0.5% Ti,max 2.5% Mo+W, max 0.5% V, max 1.25% Nb+Ta and balance of Fe (withnaturally occurring impurities) was produced by an ordinary steel makingprocess, followed by hot-rolling down to a thickness of less than 4 mm.Thereafter, it was cold-rolled in several steps with a number ofrecrystallization steps down to a final thickness of 0.2 mm. Strips offour other steel alloys were produced in the same way for comparisonwith the steel alloy of Example 1. The compositions of these additionalsteel alloys and their Sandvik identity numbers are given in Table 2.

Coupons of the five steel alloy strips with the size 70×30×0.2 mm wereoxidized in air at 850° C. for 336, 672 and 1008 hours, respectively. InFIG. 1, the mass gain per unit area is plotted as a function of time forthe five steel alloys. According to FIG. 1, a mass gain of less than 1.1mg/cm² per 1000 hours is obtained for the steel alloy of Example 1,insuring a good high temperature corrosion resistance and a lower growthrate of the formed oxide scale. However, for the steel alloys made forcomparison (Sandvik ID numbers 433, 436 and 437) with a Mn content of0.5% (by weight) and the addition of rare earth metals in the form ofCe, all showed a mass gain of more than 1.8 mg/cm² per 1000 hours, andthe steel alloy with a Mn content of 5% (by weight) had a mass gain ofalmost 5 mg/cm² per 1000 hours. The extreme large weight gain for steelalloy with 5% Mn (Sandvik ID number 434) shows the importance of goodmonitoring of the Mn content in the alloy to avoid high temperaturecorrosion. A conclusion to be drawn from this is that the Mn content inthe steel alloy should be carefully monitored and low amounts of Mn asalloying element is preferred if good high temperature corrosionresistance is to be obtained.

In FIG. 2, a cross sectional Scanning Electron Microscopy (SEM)micrograph of the formed oxide scale after 336 hours at 850° C. in airon the steel alloy of Example 1 is shown. In FIG. 2, it can be seen thatthe formed oxide scale is also well adherent to the underlying steelalloy and that the oxide scale thickness is less than 3 μm.

The chemical composition of the formed oxide scale after oxidization inair for 336 hours at 850° C. was determined by Glow Discharge OpticalEmission Spectroscopy (GDOES). In FIG. 3, the GDOES depth profile forthe formed oxide scale is shown. The different scales for differentelements should be noted. In FIG. 3, it can be seen that the manganesecontent in the formed oxide scale increases at the surface to about 12%by weight. The thickness of this manganese-rich oxide scale is about 0.5μm, followed by a more chromium-rich oxide scale of approximately lessthan 2.4 μm. The formation of a manganese-rich oxide scale at anoutermost layer at the surface is of importance since ternary chromiumoxides such as MnCr₂O₃ are believed to lower the formation of volatilechromium species such as chromium oxyhydroxides. It can also be seenthat the titanium content is approximately 0.4% by weight in the oxidescale. Finally, it can be noted that at the interface of the steel alloyand the oxide scale, a region of silicon oxide is obtained. Theformation of silicon oxide should be kept as low as possible but isunavoidable if the steel is alloyed or has small residuals of silicon inthe matrix. However, as long as the formation of insulating siliconoxide at the steel interface is only as small islands of particles, andnot as a continuous layer, it is acceptable for the performance of thefuel cell. X-ray diffraction of the oxidized coupon showed that theoxides formed in the scale had both spinell (MCr₂O₃) and corundum (M₂O₃)types of structures.

EXAMPLE 2

As an additional example of an exemplary embodiment of the disclosedsteel alloy, coupons of the final steel alloy strip with the sizes ofapproximately 30×40×0.057 mm were oxidized in air at both 750° C. and850° C. for 500 and 1000 hours, respectively. In Table 3, a summary ofoxidization results of the four samples together with the exact couponsizes of the initial samples is given. The result at 750° C. showed avery low mass gain per unit area, lower than 0.2 mg/cm² after 500 hoursof oxidization and the mass gain did not increase greatly after 1000hours. Instead, it was still lower than 0.3 mg/cm² after 1000 hours. Forthe two sample oxidized at 850° C., the mass gain per unit area waslarger but still low, less than 1.1 mg/cm², which was also the resultfor the thicker strip (0.2 mm) samples, oxidized in Example 1.

The low mass gains observed in both Examples 1 and 2 above were comparedwith published values of mass gain attained on other commerciallyavailable steels, and other test melts. In Table 4, values as obtainedfrom the literature on other steel grades together with the valuesobtained in the present study have been summarized for comparison withan embodiment of the present invention. In Table 4, it can be seen thatthe weight gain of the steel alloy of exemplary embodiments of thedisclosed steel alloy is low compared to other commercially availablesteel grades. For instance, it has been reported that the commerciallyavailable steel ZMG232 has a weight gain of approximately 0.5 mg/cm²,already after 100 hours exposure of air at 850° C. The same alloy whenexposed to air +1% H₂O mixture for only 670 hours at 850° C. has an evenlarger weight gain of 1.54 mg/cm².

EXAMPLE 3

As a third example, coupons of exemplary embodiments of the disclosedsteel alloy and the four test melts (Sandvik ID numbers 433, 434, 436and 437) described in Example 1 and the Sandvik 0C44 alloy, wereoxidized together with coupons of two commercially available steelgrades designed for the use as interconnects in SOFC, alloy A and alloyB at 850° C. in air +1% H₂O for 500 hours. In FIG. 4, the weight gainfor the different steel grades after oxidation at 850° C. in air +1% H₂Ois shown. In FIG. 4, it can be seen that the exemplary embodiments ofthe disclosed steel alloy have a much lower weight gain compared withthe four Sandvik test melts, and also have a much lower weight gain thanthe two commercially available steel grades. In this context, a lowweight gain is equal to a good high temperature corrosion resistance.The second lowest weight gain is obtained by the Sandvik 0C44 alloy withthe nominal composition (by weight) of max 0.018% C, max 0.025% N, max0.5% Si, max 0.35% Mn, 21.1 to 21.8% Cr, max 0.3% Ni, max 0.02% P, max0.007% S, max 0.15% Mo, max 0.010% Ti, max 0.01% Nb, max 0.03% Ce, max0.015% Mg and balance of Fe (with naturally occurring impurities). Asseen in Example 1, the steel alloy Sandvik ID number 434 with a high Mncontent has the largest weight gain almost 3 mg/cm² after 500 hours ofexposure. The commercial available alloy B with a composition of (byweight) C=0.02%, Si=0.40%, Mn=0.50%, Ni=0.26%, Cr=21.97% Al=0.21%,Zr=0.22%, La=0.04% and balance of Fe according to reference “Developmentof Ferritic Fe—Cr Alloy for SOFC separator”, T. Uehara, T. Ohno & A.Toji, Proceedings Fifth European Solid Oxide Fuel Cell Forum, Lucerne,Switzerland, edited by J. Huijsmans (2002) p. 281, has the secondlargest weight gain of almost 2.5 mg/cm² after 500 hours of exposure.The three other Sandvik steel alloys ID#433, 436 and 437 with only 0.5%by weight Mn and the commercial available alloy A with a nominalcomposition of (by weight) Cr 21.0 to 24.0%, C max 0.03%, Mn max 0.8%,Si max 0.5%, Cu max 0.5%, Ti max 0.25, P max 0.05, La max 0.2% andbalance of Fe, have weight gains of less than 1 mg/cm², but still muchhigher than exemplary embodiments of the disclosed steel alloy and theSandvik 0C44 alloy.

EXAMPLE 4

All the three previous examples have described the excellent hightemperature corrosion resistance of exemplary embodiments of disclosedsteel alloy. In this fourth example, the low electrical resistivity ofthe exemplary steel alloys will be exemplified. The contact resistancewas measured in dry air for 2900 hours at 750° C. with a temperaturepeak of 850° C. for 10 hours in the beginning. The load of the contactwas 1 kg/cm² at the start and the contact area was 0.5 cm². The measuredarea specific resistance (ASR) was initially, i.e., after the 850° C.temperature peak lower than 15 mΩ cm² and had after 2900 hours,including 6 thermal cycles, increased to below 25 mΩ cm². The incrementof the ASR with time was lower than 5 mΩ cm² per 1000 hours. Ifextrapolated linearly, the ASR of the contact would be less than 200 mΩcm² after 40,000 hours of exposure. It is important for the fuel cellefficiency that the ASR is low throughout the lifetime of the fuel cell.Furthermore, when the contact resistance was tested under anode gasenvironment, i.e., Ar+9% H₂ at 750° C., the ASR was even lower, wellunder 10 mΩ cm² after 600 hours of exposure. The increment of the ASRwas very low, under 2 mΩ cm² per 1000 hours. If extrapolated linearlythe ASR of the contact on the anode side would be much lower than 200 mΩcm² after 40,000 hours of exposure, even lower than 100 mΩ cm² after40,000 hours of exposure. These values can be compared with the contactresistance of approximately 26 mΩ cm² after exposure of air at 750° C.for 1000 hours for the commercially available steel ZMG 232 which havebeen reported in “Development of Ferritic Fe—Cr Alloy for SOFCSeparator”, T. Uehara, T. Ohno & A. Toji, Proceedings Fifth EuropeanSolid Oxide Fuel Cell Forum, Lucerne, Switzerland, edited by J.Huijsmans (2002) p. 281. TABLE 1 Area Oxide Scale Volume Weight of theWeight gain (cm²) Thickness (μm) (cm³) scale (mg) (mg/cm²) 1 1 0.00010.52 0.16 1 2 0.0002 1.04 0.33 1 5 0.0005 2.60 0.82 1 10 0.0010 5.201.64 1 1.4 0.00014 0.73 0.23

TABLE 2 Sandvik ID # C Si Mn P S Cr Ni Mo Nb V Al Ce N 433 0.008 0.160.55 0.005 <0.001 22.37 0.11 <0.01 <0.01 0.013 0.040 0.11 0.029 4340.009 0.14 5.06 0.004 0.001 22.23 0.11 <0.01 <0.01 0.012 0.014 0.0810.028 436 0.007 0.15 0.52 0.005 <0.003 22.27 1.04 <0.01 <0.01 0.0130.039 0.12 0.029 437 0.009 0.14 0.50 0.004 0.001 22.39 2.96 <0.01 <0.010.014 0.020 0.05 0.032

TABLE 3 Size Thickness Mass Time Temp. Mass after Gain Sample mm² mmgrams hours ° C. grams mg/cm² 1 29.5 × 40.0 0.058 0.5278 500 750 0.53110.14 2 30.0 × 40.0 0.057 0.5256 1000 750 0.5322 0.27 3 30.0 × 39.0 0.0570.5096 500 850 0.5290 0.83 4 30.0 × 40.0 0.056 0.5219 1000 850 0.54661.03

TABLE 4 Temperature Time Weight gain Steel/Supplier ° C. (hr) Atmospheremg · cm⁻² Reference ZMG 232 - Hitachi 750 100/1000 Air 0.18/0.36 1 850100 Air 0.5 1 1000 100 Air 2.4 1 JS-3 Julich-KTN 800 500/1000 Air0.25/0.32 2 JS-1 Julich-KTN 800 500/1000 Air 0.78/1.03 2 ZMG 232 -Hitachi 750 670 Air + 1% H₂O 0.27 3 800 670 Air + 1% H₂O 0.73 3 850 670Air + 1% H₂O 1.54 3 900 670 Air + 1% H₂O 1.95 3 JS-3 Julich-KTN 750 670Air + 1% H₂O 0.38 3 800 670 Air + 1% H₂O 0.67 3 850 670 Air + 1% H₂O 2.03 900 670 Air + 1% H₂O 2.6 3 Steel of the 850 1008 Air 1.05 Example 1invention Sandvik ID# 433 850 1008 Air 1.87 Example 1 Sandvik ID# 434850 1008 Air 4.94 Example 1 Sandvik ID# 436 850 1008 Air 1.98 Example 1Sandvik ID# 437 850 1008 Air 2.25 Example 1 Steel of the 750 1000 Air0.27 Example 2 invention Steel of the 850 1000 Air 1.03 Example 2invention Steel of the 850 500 Air + 1% H₂O 0.22 Example 3 inventionSandvik ID# 433 850 500 Air + 1% H₂O 0.72 Example 3 Sandvik ID# 434 850500 Air + 1% H₂O 3.0 Example 3 Sandvik ID# 436 850 500 Air + 1% H₂O 0.80Example 3 Sandvik ID# 437 850 500 Air + 1% H₂O 0.92 Example 3 Alloy A850 500 Air + 1% H₂O 0.90 Example 3 Sandvik OC44 850 500 Air + 1% H₂O0.3 Example 3 Alloy B 850 500 Air + 1% H₂O 2.4 Example 3Notes to Table 4:1. “Long term oxidation behaviour and compatibility with contactmaterials of newly developed ferritic interconnect steel”, J.Pirón-Abellán, F. Tietz, V. Shemet, A. Gil, T. Ladwein, L. Singheiser &W. J Quadakkers, Proceedings Fifth European Solid Oxide Fuel Cell Forum,Lucerne, Switzerland, edited by J. Huijsmans (2002) p. 248.2. “Development of Ferritic Fe—Cr Alloy for SOFC separator”, T. Uehara,T. Ohno & A. Toji, Proceedings Fifth European Solid Oxide Fuel CellForum, Lucerne, Switzerland, edited by J. Huijsmans (2002) p. 281.3. “Corrosion Behaviour of Chromium Steels for Interconnects in SolidOxide Fuel Cells” T. Fich Pedersen, P. B. Friehlin, J. B.Bilde-Sörensen, S. Linderoth, presented at the conference “CorrosionScience in the 21^(st) Century” held at UMIST in July 2003.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without department from thespirit and scope of the invention as defined in the appended claims.

1. A high temperature corrosion resistant stainless steel, consistingessentially of (by weight): C≦0.2%, but more than zero; N≦0.1% but morethan zero; O≦0.1% but more than zero; Si≦0.4% but more than zero;Al<0.5% but more than zero; Mn≦0.5% but more than zero; Cr 20 to 25%;Ni≦2.0% but more than zero; Zr+Hf 0.001 to 0.1%; Ti≦0.5% but more thanzero; Mo+W≦2.5% but more than zero; Nb+Ta≦1.25% but more than zero;V≦0.5% but more than zero; and balance of Fe and naturally occurringimpurities and not more than 0.010% of S impurity.
 2. The stainlesssteel according to claim 1, wherein the stainless steel has a weightgain of less than 1.5 mg/cm² when oxidized in air or air +1% H₂O at 850°C. after 1000 hours, without any spallation of the oxide scale.
 3. Thestainless steel according to claim 2, wherein the stainless steel has alinear thermal expansion coefficient of more than 11.5×10⁻⁶(° C.⁻¹) butless than 13.0×10⁻⁶(° C⁻¹) in the temperature range of 30 to 900° C. 4.The stainless steel according to claim 1, wherein the stainless steelhas a linear thermal expansion coefficient of more than 11.5×10⁻⁶(°C.⁻¹) but less than 13.0×10⁻⁶(° C.⁻¹) in the temperature range of 30 to900° C.
 5. The stainless steel according to claim 1, wherein thestainless steel has a thermal expansion mismatch (TEM) with anelectro-active ceramic material in a solid oxide fuel cell of less than±15%.
 6. The stainless steel according to claim 1, wherein the stainlesssteel is suitable for application as an interconnect or a bipolar platematerial in Solid Oxide Fuel Cells.
 7. The stainless steel according toclaim 1, wherein the stainless steel is suitable for application as acatalytic converter in automobile applications.
 8. The stainless steelaccording to claim 1, wherein the stainless steel has an Area SpecificResistance of less than 50 mΩ cm² after 1000 hours on both an anode sideand a cathode side of a Solid Oxide Fuel Cell.
 9. The alloy according toclaim 8, wherein an increment of the Area Specific Resistance is notgreater than 10 mΩ cm² per 1000 hours.
 10. A solid oxide fuel cellcomprising the stainless steel according to claim
 1. 11. A catalyticconverter intended for the automobile industry, comprising the stainlesssteel according to claim
 1. 12. A high temperature corrosion resistantstainless steel consisting essentially of (by weight): C≦0.1% but morethan zero; N≦0.1% but more than zero; O≦0.1% but more than zero; Si≦0.4%but more than zero; Al<0.4% but more than zero; Mn≦0.4% but more thanzero; Cr 20 to 25%; Ni≦1.0% but more than zero; Zr 0.001 to 0.1%;Ti≦0.4% but more than zero; Mo≦2.5% but more than zero; Nb≦1.25% butmore than zero; V≦0.1% but more than zero; and balance of Fe andnaturally occurring impurities and not more than 0.010% of S impurity.13. The stainless steel according to claim 12, wherein the stainlesssteel has a weight gain of less than 1.5 mg/cm² when oxidized in air orair +1% H₂O at 850° C. after 1000 hours, without any spallation of theoxide scale.
 14. The stainless steel according to claim 13, wherein thestainless steel has a linear thermal expansion coefficient of more than11.5×10⁻⁶(° C.⁻¹) but less than 13.0×10⁻⁶(° C.⁻¹) in the temperaturerange of 30 to 900° C.
 15. The stainless steel according to claim 12,wherein the stainless steel has a linear thermal expansion coefficientof more than 11.5×10⁻⁶(° C.⁻¹) but less than 13.0×10⁻⁶(° C.⁻¹) in thetemperature range of 30 to 900° C.
 16. The stainless steel according toclaim 12, wherein the stainless steel has a thermal expansion mismatch(TEM) with an electro-active ceramic material in a solid oxide fuel cellof less than ±15%.
 17. The stainless steel according to claim 12,wherein the stainless steel is suitable for application as aninterconnect or a bipolar plate material in Solid Oxide Fuel Cells. 18.The stainless steel according to claim 12, wherein the stainless steelis suitable for application as a catalytic converter in automobileapplications.
 19. The stainless steel according to claim 12, wherein thestainless steel has an Area Specific Resistance of less than 50 mΩ cm²after 1000 hours on both an anode side and a cathode side of a SolidOxide Fuel Cell.
 20. The stainless steel according to claim 19, whereinan increment of the Area Specific Resistance is not greater than 10 mΩcm² per 1000 hours.
 21. A solid oxide fuel cell comprising the stainlesssteel according to claim
 12. 22. A catalytic converter intended for theautomobile industry, comprising the stainless steel according to claim12.